The invention relates to a method for preparing coarse-crystal or single-crystal metal films, wherein a metal is precipitated as a layer on a suitable substrate by vapor deposition or atomizing.
In electrical engineering and in particular, in semiconductor technology, thin metal films are used for conductors, electric resistors or as capacitor electrodes. These components must exhibit very long life and their electrical properties must be highly stable under varying thermal stresses. Thus, such metal films must withstand, for example, short-time temperature stresses of up to about 400.degree. C. as well as continuous temperature rises to approximately 150.degree. C. without change of their properties.
Such metal films are usually prepared by vapor deposition or sputtering on substrates such as glass, silicon or ceramic material. These metal layers are generally of fine-crystalline structure. Thus, tantalum, for example, which is vapor-deposited or sputtered-on at a substrate temperature of 150.degree. C., has a grain size of about 10 nm. The property of the films in having a fine-crystalline structure can result in various disadvantages depending on the eventual application of the films. If components provided with such metal layers are, for example, operated at elevated temperatures, then grain growth can occur in these fine-crystalline metal layers, which considerably changes their properties, for example, the electric resistance as well as the temperature coefficient of the electric resistance. If such fine-crystalline metal films are used as conductors, heavy electro-migration can occur in such conductors at the high current densities required in highly integrated circuits (cf. "Proc. of the IEEE", vol. 59, no. 10 (Oct. 1971), pages 1409 to 1418). The essential cause of electromigration is structural inhomogeneities in the material of the conductor, such as grain boundaries. In order to obtain stable properties of such metal films over a greater temperature range, metal films have heretofore been subjected to an extended heat treatment which, as a rule, leads to grain growth within the layers. Such an annealing process, however, has its problems especially with semiconductor circuits since other, already formed components can be destroyed by the high temperatures required for the heat treatment. Especially in narrow conductors, growth of the grain boundary surfaces across the entire conductor can be caused by such an annealing process. If a current load is applied to the conductor, this in turn leads to increased electromigration at these points, which in turn leads to a failure of the conductor and thereby, of the component. It is desirable to find for the conductors a material which does not exhibit such electromigration. This could be achieved by very coarse-crystal or single-crystal metal layers.